I have Verizon and have used the $10/day plan overseas, including in Ireland.
The Travel Pass plan works the way that they say it does. For $10/day, you can use all of the existing features of your home plan - calls, texting, data - just like you would at home. The day's session starts the moment that you make your first call, text, etc., and it expires 24 hours later. They send you a free text that tells you that your 24 hours is about to expire. I think the rate is slightly less for travel to Canada and Mexico, but for Ireland it would be $10/day.
If you don't plan to use the phone much, it's not a bad deal. If you can get by on hotel WiFi and don't need to constantly reach out and touch someone, then it's a good emergency system.
Verizon also has a $40/month international plan with different features and different limitations. I haven't used that specific one, so I don't know how much data that includes, how many calls, if texts are included or priced separately, etc.
Either way, you have to call Verizon or log into your account online to arrange international service before you leave. I know that Travel Pass can be set up online. I don't know about the $40/month plan.
If you do go with the $40/month plan, you'll need to call to cancel it when you return. Otherwise it will remain on your bill as a recurring feature. (At least that's how it used to work when I used their monthly travel plans for overseas trips.)
Travel Pass is only charged when it is used. It also remains on your account until you call or go online to remove it, but again you are only charged for it when you are outside the US.